Induction motors are used in many applications for rotating a motor shaft in a direction or in the opposite direction at a certain speed. For example, one application may include using induction motors for moving an arm carrying read/write heads of a hard disk drive (HDD) from a parked position to a position above the disk and vice versa. In these applications, it may be important to precisely control the motor speed to prevent the arm carrying the read/write heads from hitting against a run stop when they are moved off the disk to the parking ramp, preventing possible damage of the read/write heads.
A typical approach for monitoring the speed of an induction electric motor is to sense the back electromotive force (BEMF) induced in the primary winding of the motor by the moving rotor. To this end, the BEMF is sensed with a circuit as depicted in FIG. 1, commonly referred to as “BEMF Amplifier.” The equivalent circuit of the induction electric motor is schematically shown within the dashed lines and comprises a generator of BEMF, an inductance Lm, and a resistor Rm of the primary winding. Typically, a sense resistor Rs is connected in series to the primary winding, and the voltage drop on this resistor corresponds to the current IM flowing through the motor. The motor is connected to a positive voltage supply line VCMP and to a negative voltage supply line VCMN.
The BEMF Amplifier senses voltage drops on the terminals of the motor and on the sense resistor Rs, amplifies the latter through an adjustable gain amplifier A1, and generates a difference signal between the voltage on the terminals of the motor and the amplified voltage. An output signal VOUT is generated as amplified replica of the difference signal by a second gain amplifier A2.
If the motor is driven in DC mode, the contribution given from the inductance Lm to the voltage drop on the terminals of the motor is negligible and the output signal VOUT is:VOUT=(BEMF+IM·Rm−IM·Rs·A1)·A2  (1)By adjusting the value of the gain A1 such to satisfy the following equation:A1=Rm/Rs  (2)results in:VOUT=BEMF·A2  (3),that is, the output signal is an amplified replica of the back-electromotive force in the motor. Therefore, the signal VOUT represents the motor speed and does not depend on the current IM flowing through the motor.
This characteristic of the signal VOUT is particularly useful to control the motor of a HDD or a similar system when the supply line is unintentionally interrupted, situation commonly referred to as “power down” condition. When a power down condition occurs, it may be necessary to park the read/write heads to prevent them from staying over the disk, thus preventing any risk of damaging the surface of the disk with a consequent loss of data. The fact that the signal VOUT does not depend on the current IM makes it track the BEMF both during the normal functioning as well as when the supply line of the motor is unintentionally interrupted.
The equivalent resistance of the motor Rm is not known a priori and varies with the functioning temperature, thus it may be necessary to adjust the gain A1 such to satisfy equation (2). Indeed, this adjustment is carried out with the rotor locked (BEMF=0) in a certain position. In these conditions, it is possible to supply current to the motor and ensure that the motor may not start running. The adjustment includes varying the gain A1 until the signal VOUT becomes null. When this occurs, the equation (2) is satisfied.
As stated hereinbefore, the equivalent resistance Rm of the motor varies during its functioning, thus the signal VOUT may not reliably track the BEMF. This may lead to an inaccurate determination of the motor speed. This problem is even more disadvantageous in the case of power down conditions. When a power down occurs, for example, in a HDD, it may be necessary to park the read/write heads and to control accurately the speed of the arm moved by the motor.
If equation (2) is no longer verified because the equivalent resistance Rm of the motor has varied, it may be necessary to repeat the step of determining the gain A1. This is not possible with the previously described technique because in general the condition BEMF=0 is not verified and it is likely not possible to be sure that the rotor is locked to force a current through the motor without moving it.